From the album The Great Divide
Kahan is building an entire identity around someone who left, while insisting he's moved on. Every act of self-improvement is still about them. The running, the lifting, the tattoo of their initials on his right arm so he sees their name every time he drinks. He calls it healing but the song is a shrine.
You broke in and stole china from our livin' room / Kinda makes all the other parts boring
This person's chaos is so large it makes everything else in the story disappear. The breakin becomes the only anecdote worth telling, which means this relationship is still the most interesting thing about his life.
I'd beat your ass 'til the morning / You know I've been running all this time
The fantasy switches from violence to cardio without warning. Both are ways to prove he's gotten stronger, but neither erases the fact that the person he's comparing himself to isn't even here to see it.
Tattooed your initials into my right arm / So I'd see your name when I lift up a drink
He literally carved them into his body in a way that guarantees permanent visibility during the exact activity he uses to cope. Moving on would mean not needing this reminder at all.
I've got a feeling that won't go away / The doctors are calling it 'just moving on'
The medical language turns grief into diagnosis. He knows what it is but refuses to call it that, because naming it correctly would mean admitting the feeling is normal instead of something only this person could cause.
Twenty-three, clean in the engine heat / It can all be the way that it was
The tense collapses. He wants them frozen at 23 and sober forever, which means the fantasy isn't about reunion but about time travel. He doesn't want the real person back. He wants the version that never hurt him.
Kahan would be shocked to realize his entire fitness routine, his social persona, his drinking ritual, are all still organized around this person. The song thinks it's about getting stronger. It's actually about building a life-shaped monument to someone who isn't coming back.